1. Field of the Invention
The present invention relates to a control device for a motor that is mounted in a vehicle.
2. Description of the Related Art
A motor control device for an on-vehicle motor usually includes a microcomputer. The microcomputer calculates a command current value that corresponds to a required torque and controls a bridge circuit of power switching elements in a pulse width modulation (PWM) control mode so as to make the difference between the command current value and the amount of detected current become zero. In this method, there is such a problem that the microcomputer may have a higher possibility to go out of control than a discrete control device if it is excessively heated.
On the other hand, the microcomputer may have to be located near the power switching elements that is mounted on the same circuit board in order to make the device compact. Therefore, microcomputer is subject to a high temperature of the power switching elements.
In case of an electric motor power steering system (EPS) that provides a driver with assisting torque, the amount of current to be supplied to a motor is calculated by a CPU according to required steering torque. If the CPU goes out of control due to an excessively high temperature, it may provide a wrong assisting torque.
JP-A-2002-67988 and JP-A-2003-335251 disclose electric motor power steering systems that can prevent the above problem by stopping the motor. However, the motor is stopped as soon as the computer goes out of control. Accordingly, the assisting torque suddenly disappears. This may frighten a driver or may give the driver a considerable shock.